ActivityIndicator upgrade

**Is your feature request related to a problem? Please describe.**
`ActivityIndicator is limited in customization and naming compared to modern UI needs.

**Describe the solution you'd like**
- Rename `ActivityIndicator` to `Spinner`
- Add more spinner variants

**Describe alternatives you've considered**
Using custom loaders or third-party spinner libraries.

**Additional context**
A Spinner component with multiple variants would improve flexibility and align better with Paper theme.

Start by locating the existing ActivityIndicator component and any related tests or exports in react-native-paper, then review the project's component and theme conventions. Clarify the Spinner API and the intended variants before implementation; done means the component is renamed, multiple variants are supported, and affected usage and tests are updated.

Written by the indexing model from the issue text.